1. Field of Invention
This invention is directed to a new class of bifunctional blue emitting materials for organic electroluminescent (EL) application.
2. Discussion of Related Art
It is desirable to have organic EL devices (OLED) provide uniform luminescence, saturated color especially in the blue regions of the visible spectrum, low driving voltages, and that maintains device efficiency.
A typical organic EL device can be comprised of a layer of an organic luminescent material conductively sandwiched between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. A number of prior art organic EL devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene, as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325. However, these devices usually require excitation voltages on the order of 100 volts or greater.
In U.S. Pat. No. 4,539,507 there is disclosed an EL device formed of a conductive glass transparent anode, a hole transporting layer of 1,1-bis(4-p-tolylaminophenyl)cyclohexane, an electron transporting layer of 4,4′-bis(5,7-di-tert-pentyl-2-benzoxzolyl)stilben, and an indium cathode.
U.S. Pat. No. 4,720,432 discloses an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport.
U.S. Pat. No. 4,769,292 discloses an EL device employing a luminescent zone comprised of an organic host material capable of sustaining hole-electron recombination and a fluorescent dye material capable of emitting light in response to energy released by hole-electron recombination. A preferred host material is an aluminum complex of 8-hydroxyquinoline, namely tris(8-hydroxyquinolinate)aluminum.
For visual display applications, organic luminescent materials should provide a satisfactory color in the visible spectrum, normally with emission maxima at about 460, 550 and 630 nanometers for blue, green and red. These organic EL devices may comprise a light-emitting layer which is comprised of a host material doped with a guest fluorescent material that is responsible for color emission. For efficient down-shifting of EL emission wavelength in the host-guest emitting layer, it may be desirable that the host material should fluorescence in the blue or shorter wavelength region.
In many conventional organic EL devices, the luminescent zone or layer is formed of a green-emitting luminophor of tris(8-hydroxyquinolinate)aluminum with certain fluorescent materials. U.S. Pat. No. 5,409,783 discloses a red-emitting organic EL device by doping the tris(8-hydroxyquinolinate)aluminum layer with a red fluorescent dye. However, up-shifting of the tris(8-hydroxyquinolinate)aluminum emission to blue region is believed to be highly inefficient. Although there have been several disclosures describing blue-emitting organic EL devices, for example in U.S. Pat. Nos. 5,151,629 and 5,516,577, the disclosures of which are fully incorporated herein by reference, their performance characteristics still possess many disadvantages such as poor emission hue, high operation voltages, low luminance, and poor operation stability.
Additionally, while recent progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the performance levels of a number of current available devices, especially with respect to blue emission, may still be below expectations. In a typical organic EL device containing three layers, with hole-transporting and emitting and electron-transporting properties, when voltage is applied, the holes migrate through the hole transporting material and loosely bond with the electrons at the interface with the electron transporting layer. When holes and electrons recombine they form excitons that are responsible for the emission of light. The location of the recombination zone depends on many factors such as the energy alignment of the layers and mobility of the holes and electrons. Because the carrier mobility of the holes in the hole transporting layer is usually greater than that of the electrons in the electron transporting layer, some percentage of the holes can leak up to the cathode if the electron transporting layer exhibits a matching highest occupied molecular orbital level. This phenomenon could dramatically decrease the efficiency of the device.